Why do photos lie and how not to get misled

I don't know if there is another topic as complex as the hooves.

To understand them well, you need not only to have mastered all the knowledge regarding horses,  including their anatomy, physiology and behavior. You should be familiar with issues related to medicine, biology, biochemistry, biomechanics, mechanics, material properties, and it turns out that geometry and optics are important as well.

Currently, photography is one of the basic tools used to document hooves’ condition. Oftentimes, digital photos are used to measure angles, hoof proportions and to compare progress (or regression) before and after trimming, and over longer periods of time.

It’s fair to say that documenting hooves with photography has become a kind of a standard. Therefore, some guidelines for taking photos have been set. For example - when taking side shots, to make sure that the camera is parallel to the hoof, it is sometimes advised that the camera beam is directed the way that one heel bulb perfectly hides behind the other. Some recommend that the lens is at or just above the ground. Others say it’s necessary to put the horse on blocks and direct the beam at the base of the sole. Occasionally, someone will mention that you should back up from the hoof and zoom in, but usually without explaining why. Sometimes it is recommended to take several shots, one close up and another one from a distance, in order to also be able to assess the hoof-pastern axis.

All those recommendations do make sense, but this is far too little to consider the image captured in the photo (let alone the measurements taken) as reliable. I feel that true understanding of how important it is to take and interpret photos consciously is still missing.

But how much can we go wrong?

So what do you need to take into account in order to take shots that reflect reality well - and to be able to spot and understand what causes the distortions that occur in the photos?

I've distinguished 3 major phenomena that contribute to the distortions we see in the photographs:

1.  PERSPECTIVE
2.  LENS DISTORTION
   
3.  DEFORMATIONS DUE TO SHIFT OF THE PHOTOGRAPHED OBJECT IN RELATION TO THE CAMERA MATRIX CENTER- caused by both of the above

PERSPECTIVE

Perspective is the main reason of why photos ‘lie’. There are 3 major factors that it consists of:

• distance between the camera and the object
  
• angle between the camera and the object
  
• distance between the camera and the ground

DISTANCE BETWEEN THE CAMERA AND THE OBJECT

Photos of the same hoof taken within seconds at the same height and the same angle. The hoof and the horse's posture have not changed in between.

The only difference is the distance between the camera and the hoof. Note the difference in the look of the heel bulbs - the horse had sheared heels and the medial was higher than the lateral one. This is visible from further distance.

The shorter the camera - hoof distance:

• the more distortions - objects and parts of objects closer to the camera seem to be larger -> distortion of the proportions

• the less hoof capsule captured in the photo

This set of photos illustrates analogical distortion related to the distance, affecting the hoof and the man’s face. The closer you get, the more change in proportions - elements closer to the camera seem bigger than they really are, making some of the parts further away invisible – like the shoe clip or man’s ears.

The visible surface of the hoof captured from different distances.

The real location of the ‘dorsal wall’ (the most dorsal part of the wall visible to the camera) one would see in photos taken from different distances.*

*Hint: you can use the shoe clips and nail heads as indicators of how much hoof you are actually seeing in the picture.

In general – the shorter the hoof-camera distance, the less hoof surface is being captured in the photo and the more distorted the proportions are. This is simply due to the rules of perspective – objects closer to the camera seem to be bigger than the ones further away. As a result of these phenomena, the angles and proportions get distorted, likely making all the measurements to-be-taken unreliable. But measuring the dorsal wall angle may actually become not possible – as in most cases we don’t even see the dorsal wall! In my tests it turned out that, depending on hoof size and geometry, (with camera parallel to the hoof and centered) the dorsal wall was (nearly) captured in the photo only at distances around 500 cm!

To get more accurate data, I marked 2 different hooves with lines parallel to the tubules in approximately 1 cm increments and took a set of photos.

Approximate distance between the last visible tubule and the center of the dorsal hoof wall.

And how does that affect the ‘dorsal wall’ angle and balance in reference to COR?

All these changes occur with only the change of the hoof-camera distance, nothing else!

The same rules apply also when photographing the hooves from any other direction. By placing the camera behind the same hoof in 10 cm and 100 cm distance, I got following results:

Photos of the same hoof taken within seconds. The only change was the camera distance to the hoof. Uncropped version below:

So what should we do? What is the best distance for taking good photos?

In general: geometrically, the further away you place the camera from the hoof, the better. But you don’t need and probably won’t be able to back up by 5 m each time! Every camera has different focal length and resolution and usually moving away from the hoof makes you lose quality of the photo. So you should pick up a distance that suits you best (most preferably not closer than 70-100 cm) and stick to it, especially when taking follow up shots.

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DOES FOCAL LENGTH MATTER?

Perspective distortions are often confused for being caused by the focal length change (basically by wide angle lens). 

Unlike often thought, wide angle lens do not cause distortions by itself (other than they are more likely to create barrel lens distortion - which rarely is very significant and is explained later in the article). The problem is that wide angle lens 'encourage' you to come closer to the photographed object - which would not fit in the frame when using longer lens. In fact, most of those deformations are due to perspective distortions.

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Don’t pay too much attention to the area the hoof fills on the screen, just stick to your chosen distance. In the pictures taken with different focal lengths (but from the same distance) the hoof and background proportions will vary. Using a camera with a wider field of view may subconsciously encourage you to move closer to the hoof. Don’t! When you rescale such photos, the hoof will look exactly the same on all of them (except for the resolution of course).

A set of illustrations showing how different focal lengths affect the amount of the background captured in the photo. The red rectangle shows the area captured by the standard lens and the blue - area captured by narrow angle lens in reference to the wide and standard ones (the proportions will vary depending on the exact focal length values).

The same hoof captured with different lenses from the same distance. After rescaling, it will look exactly the same - except for the resolution:

Rescaled images. The only difference is the resolution.

The same hoof captured with different lenses from distances adequate to maintain similar hoof size on the screen. Despite similar hoof - background area ratio, the hoof looks different in each picture.

Don’t rely on measurements taken on close up photos and don’t compare photos taken from different distances with each other. That will likely lead you to wrong conclusions.

 

ANGLE BETWEEN THE CAMERA AND THE OBJECT

 The angle between the hoof and the camera can be considered in two dimensions:

• horizontally
  
• vertically 

ROTATION OF THE CAMERA AROUND THE HOOF - HORIZONTALLY

I've used the same two hooves as in the previous part of the article to check how rotating the camera horizontally around the hoof will affect what we get in the photograph. 

The same hoof captured from 3 different spots - 1. slightly from in front 2. straight from the side 3. slightly caudally. The horses posture did not change in between - note how different the HPA looks.

The same hoof captured from 3 different spots - 1. slightly from in front 2. straight from the side 3. slightly caudally. The horses posture did not change in between. Note how different the toe and heel angles and their ratios are.

Photos of the same hoof taken within seconds. Nothing has changed except for the camera placement and its angle in relation to the hoof. Note how the heel angle - toe angle ratio has changed - the toe angle has increased by 4.5° whilst the the heels by 11.5°! 

In general, the flared and collapsed heels are going to look better when captured slightly from the front (their angle will be steeper). When they are folding under, they will look worse from this view.

The hoof is a 3D object and we want to capture it in a 2 dimensional photograph. Therefore, we must be aware that what we see in each picture is just one of it’s projections and that it WILL look differently from every different view. So will it’s angles and proportions. 

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AN EXCERPT FROM DESCRIPTIVE GEOMETRY

An axonometric drawing of a regular triangular pyramid and its projections on two planes that are neither parallel to its base's sides nor to its angle's bisectors/heights. The angles marked in red are the true angles of the pyramid's edges. All of the colored angles projected on the planes have different values and none of them shows the true value of this (red) angle.

The only way to get the true angle value projected on a plane, is to place the plane perfectly parallel to the plane on which the angle is inscribed.

To capture true dorsal wall angle on a 2d view, the camera matrix should be parallel to the sagittal section of the hoof.

When camera matrix is not parallel to the sagittal section of the hoof, the measured angle value will be different from its true value - how big the difference will depend on the hoof shape, camera angle and distance.

But we already know that it's really difficult to have the true center of the dorsal wall captured in the photo due to the rules of perspective. Unless you move very far away from the hoof, you are not going to see it anyway. You can't just rotate around the hoof to make the dorsal wall visible, as it will distort the measurements. So how can we make use of the knowledge above?

To get a true angle of the dorsal wall angle captured in the photo:

1. Place the camera parallel to the sagittal section of the hoof (instructions at the end of the article)

2. Place the camera vertically to the ground (instructions at the end of the article)

3. Shift the camera in the anterior-posterior direction until the dorsal wall crosses the vertical midline of your camera frame and take the shot

Use this shot only for the dorsal wall angle measurements - the rest of the hoof is going to be distorted.

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Unless the photograph is taken within a strict regime, the measurements taken on it are not going to be reliable. But as far as the dorsal wall angle can be measured properly on a good picture, can we even talk about measuring the true angle of the heels? They are a 3d structure and one 2d view is not enough to tell their angle. You can measure it for comparisons over time but it's not going to be an absolute value. 

The same hoof captured from the side and from the back. In the side shot the heel angle looks pretty good, but you don't see how flared the heels are - which becomes obvious when looking from the back. Heels are 3d and so is their true angle. The same happens when the heel folds under the hoof:

ROTATION OF THE CAMERA AROUND THE HOOF - VERTICALLY

Similar rules apply to taking photos slightly from above. When you have a flat hoof with flares on the sides, moving the camera slightly up and to the front can make it look like much better. Opposite goes for a hoof with the heel folding under.

Pictures illustrating how rotating the camera vertically will distort the shape of the hoof. The closer the camera angle to the angle of the side wall of the hoof, the taller/deeper (and likely better) the hoof will seem. The measured angles are also going to change.

I always recommend that you place your camera so that it's perpendicular to the ground and parallel to the sagittal section of the hoof. I will explain how to do it later in the article as well as how to take photos in which you can actually measure angles and their true values. Still, if you only have and old photo taken from a different angle, just for comparison try to make the new one from similar perspective.

The bigger the hoof - camera distance, the smaller the distortions occurring due to the angle change.

DISTANCE BETWEEN THE CAMERA AND THE GROUND

Most new phones have camera lenses placed asymmetrically, in one corner. Just switching the side of the phone which we place on the ground can yield us different results in the photo.

The closer the camera to the hoof, the bigger the differences.

The same hoof captured from 2 different spots - 1. close to the ground level 2. about 8-10 cm from the ground level. The camera matrix was perpendicular to the ground in both cases. Nothing has changed in between except for the camera placement. Note the change in the coronary band shape.

In most cases the lower (and closer) the camera, the more bowed look of the coronary band.

Taking photos at different heights will especially distort the caudal views of the hoof. Placing the camera higher will make the caudal part look more collapsed and the other way round - from ground level the heel bulbs and the coronary band will look more straight.

The same hoof captured within seconds with different camera heights. Nothing has changed except for the camera position.
When trying to compare the caudal part of the hoof over time eg. to monitor caudal failure, it's especially important to take photos at the same height. Otherwise it's actually impossible to reliably compare such photos.

AND DO YOU REALLY NEED TO PUT THE HORSE ON BLOCKS TO TAKE GOOD PICTURES?

Not really, unless for some reason you want your photos to be taken from the base level or - to be more precise - lower than your camera shape allows you. That’s not necessary for any measuring purposes though and - as said above - can aggravate distortions in the coronary band.

Photos of the same hoof on a block and without it, taken on the same height in relation to the hoof. The hoof looks basically the same.

SOLE VIEW

Pictures of the sole get similar distortions as any other picture but to different degrees. They don't distorted by decreasing the hoof - camera distance as much as the side and caudal shots. But they are strongly affected by the change of the camera angle - both the symmetry, proportions, heel length and position, frog shape etc.

The same hoof captured from 3 different camera positions. Nothing about the hoof has changed in between. Note the symmetry, heels position, toe length and heel bulbs shape.

It's not necessary due to the rules of geometry but - for consistency - I always recommend to take sole shots with the camera parallel to the sole surface and place the lens in the center of the sole.

CAUDAL VIEW

Position of the camera in relation to the limb axis and sole plain will affect the distortions occurring in the photos of the caudal aspect of the hoof. I will affect the proportions, symmetry, length and angle of the heels and can even make the heel bulbs look more or less sheared than they really are. As with any other view, you should stick to a consistent protocol when taking the caudal shots.

What I like to stick to when taking caudal shots is:

• frog stay in the center of the frame and aligned vertically with the frog apex
• the sole plain and the heel aligned horizontally

When the limb has significant rotational or angular deformity, I like to take more shots - one aligned with the limb axis, one aligned with the hoof capsule.

LENS DISTORTION

Also known as optical distortion, occurs due to optical design of the lens. In short - it's an optical aberration that makes physically straight lines appear curvy in the image. It's something that the photographer has no influence on (other than the choice of the lens) but I think it's important to be aware of the phenomena and how it affects the image.

1. Image without distortion   2. Barrel distortion   3. Pincushion distortion   4. Fisheye distortion

Image with no optical distortion and two main types of lens distortions - barrel distortion and pincushion distortion. The last image shows a fisheye distortion which is used in ultra-wide lenses and is an extreme example of barrel distortion.

Photographs of the same hoof taken at similar distance and angle but with different lenses - the first one was taken with the fisheye lens, the second with regular lens with slight barrel distortion. (The position of the left leg has not changed in between taking those photos but the horse has moves the right one.)

Barrel distortion is most common in smartphone cameras and in wide angle lens (most smartphones have wide angle lens in the primary camera). The pincushion distortion occurs in telephoto lenses. Zoom lenses often gradually go from barrel distortion at the shortest focal lengths to pincushion distortion at the longest end. There are lenses on the market that have those distortions almost completely corrected, they are on the expensive side, though.

What is most important to remember, is that the further away from the frame center, the more optical distortions - both barrel and pincushion. And usually that the further away from standard lens length (which means the wider or narrower the lens angle), the more significant they become. That means that when we take a closeup shot with a wide angle lens (which is the more typical way of taking hoof photos with a smartphone I see) we actually get the most of the lens distortions!

That is another reason to back up from the hoof - and the first why we should do our best to stick to the frame center.

 

DEFORMATIONS DUE TO SHIFT OF THE PHOTOGRAPHED OBJECT IN RELATION TO THE CAMERA MATRIX

Those deformations can be really significant, as this is where both lens distortions (described above) and perspective distortions overlap. For both - the further away from the frame center, the more distortions. 

Photos of the same hoof taken within seconds. The only thing that has changed was the camera shift.

The wider the camera angle (the smaller the focal length), the more distortions towards the edge of the frame. This phenomena is called volumetric anamorphosis and is related to perspective. The further away from the matrix center, the more stretched (perspective) and bent (lens distortion) the image. To make things even more complicated - the more barrel distortion, the less perspective distortion and the other way round. 

A simple illustration of the scheme of perspective distortions increasing with distance from the frame center and a photograph of some of the UMF group members taken with wide angle lens, showing this type of distortions.

Always pay attention to have the hoof centered in the frame.

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HOW TO TAKE PHOTOS

GENERAL GUIDELINES:

• MOVE AWAY FROM THE HOOF

    - around 70-100 cm is usually right

• CAMERA MATRIX VERTICAL TO THE GROUND

• CAMERA MATRIX PARALLEL TO:

    - sagittal section of the hoof (side view)

    - heel bulbs (caudal view)

    - solar surface

• THE HOOF ALWAYS CENTERED IN THE FRAME (EXCEPT FOR THE DORSAL WALL                         MEASUREMENT PHOTO)

• SET YOUR REGIME AND STICK TO IT CONSEQUENTLY

HOW TO GET PERFECTLY VERTICAL TO THE GROUND:

1. Measure the distance from the lens to the edge of your camera/smartphone
2. Measure the same distance from the ground on the hoof wall
3. Place the camera on the ground  next to the hoof and rotate it vertically until the height measured on the hoof wall crosses the horizontal midline of your frame.

HOW TO GET PERFECTLY PARALLEL TO THE SAGITTAL SECTION OF THE HOOF:

1. Place the camera on the ground next to the hoof .
2. Move and rotate the camera around the hoof until you see that bulbs align (the medial one is perfectly hidden behind the lateral)
3. Rotate the camera (without anymore anterior-posterior shift) until the edges of the bulbs align with the vertical midline of your camera frame
4. Without anymore horizontal rotation, move the camera in the anterior direction until the hoof is in the center of the frame

Similar deformations occur in x-rays as the same rules of optics apply to them. We are going to cover it in the next article.